1. Field Of The Invention
The invention generally relates to a method of fabricating a semiconductor device, and more particularly to a method of fabricating a resistor fabricated from a polysilicon layer.
2. DESCRIPTION OF THE RELATED ART
A thin polysilicon layer plays an important role as an electrode, a material for wire lead and a resistor in manufacturing a semiconductor device. In particular, a polysilicon resistor is used as a loading element of SRAM (static RAM) and a voltage dividing element of an analog device.
A conventional method of fabricating such a polysilicon resistor includes a step for depositing a non-doped silicon layer by chemical vapor deposition, and a step for doping impurities into the silicon layer by phosphorus diffusion or ion implantation. Another conventional method of fabricating a polysilicon resistor includes a step for depositing an in-situ impurity-doped silicon layer by chemical vapor deposition in which a dopant gas is to be supplied together with a silane gas, and a step for annealing to thereby fabricate a resistor. In particular, the latter method is considered to be mainly used due to its low process cost. For instance, see Abstracts of the 20th Conference on Solid State Devices and Materials, 1989, pp. 57-60.
In this method, first, a deposition is performed for 60 minutes using a conventional low-pressure chemical vapor deposition (LPCVD) apparatus, for instance, in the following conditions:
Deposition Temperature: 570 degrees centigrade or below PA1 Degree of Vacuum: 0.5 Torr PA1 Process Gas: Monosilane (SilH.sub.4) 1000 sccm.
He-based 4% PH.sub.3 50 sccm
By the above mentioned deposition, there is fabricated a phosphorousdoped silicon layer having a thickness of 200 nanometers. Next, the silicon layer is annealed, for instance, for 30 minutes under the temperature of 900 degrees centigrade to thereby activate phosphorus to obtain a phosphorus-doped polysilicon layer. Then, the polysilicon layer is subject to a patterning step using a photoresist, and thus a polysilicon resistor is fabricated.
Though not a method of fabricating a resistor, a method of fabricating an impurity-doped polysilicon layer includes a method of doping phosphorus atoms into SIPOS (Semi Insulating Polycrystalline Silicon). For instance, see IEEE 1979 Tech. Dig. pp. 522. In this method, there is fabricated a polysilicon layer in which 44 at. % of oxygen and 0.6 at. % of phosphorus are doped, through a chemical vapor deposition process under the condition that the deposition temperature is 650 degrees centigrade and the process gas is N.sub.2 -SiH.sub.4 -N.sub.2 O-PH.sub.3.
A resistor to be used for a semiconductor device is required to have stable electrical properties. In particular, a voltage dividing element for use with an analog device is required to have a polysilicon resistor having a high accuracy because a voltage is divided in accordance with a relative resistance ratio of the resistor to an adjacent resistor. For instance, it is necessary to lower a dispersion of a relative resistance ratio (a/b) under .+-.1% between adjacent resistors A having a .OMEGA.. cm of resistance and B having .OMEGA.. cm of resistance.
The electrical properties of a polysilicon resistor are significantly affected by properties of crystal grains, such as grain diameter and crystallinity, which constitutes the resistor. For instance, the electrical resistance of a polysilicon layer is significantly varied by the decrease in the number of conductive carriers due to unbalanced distribution of dopant impurities in a grain boundary, and the decrease of carrier mobility due to grain boundary diffusion. Accordingly, it is important to keep the grain boundary density present in an electrically conductive region to be uniform as much as possible in order to fabricate a resistor made of polysilicon and having stage electrical properties. In other words, it is important to arrange the number of crystal grains present in polysilicon resistors to be identical to one another. However, since the number of crystal grains has always a statistical dispersion, the dispersion in the number of crystal grains is increased with the absolute number of crystal grains being decreased.
With the increasing integration of a semiconductor device, a o resistor size is decreased accordingly. Thus, a diameter of a crystal grain of polysilicon fabricated in accordance with a conventional method is close to the size of the resistor. For instance, a resistor size is a few micrometers whereas the diameter of a crystal grain is hundreds of nanometers. Consequently, the uniformity of resistance of a resistor fabricated in accordance with a conventional method deteriorates with the increasing integration of LSIs.
The above mentioned method in which phosphorus atoms are doped into SIPOS aims to enlarge a band gap of a semiconductor, however, a polysiIicon layer fabricated in accordance with this method includes tens of percent of oxygen atoms, resulting in that the polysilicon layer has a high resistivity to thereby make it difficult to apply the polysilicon to a resistor.